The invention relates to a process for producing a thermoplastic moulded part, which comprises the following steps:
a) dispersing reinforcing fibres in a melt of a polyolefin or a polycondensation polymer,
b) injecting the polymer composition thus obtained into a closed mould by means of an extruder or an injection-moulding machine.
Such a process is known from JP-A-5-17631. JP-A-5-17631 describes a process for injection-moulding a flat plate of fibre-reinforced polypropylene.
JP-A-7-16933 discloses a similar process; the specific modulus and specific strength of a fiber reinforced article so made needs improvement.
A drawback of the method described in JP-A-5-17631 is that the fibre-reinforced polypropylene described herein results in a specific modulus and a specific strength which are lower than desired.
The aim of the invention is to eliminate this drawback This aim is achieved according to the invention in that the melt is injected through a nozzle into the mould, the mould is partly opened when at least part of the surface of the moulded part has cooled to below the softening temperature of the polyolefin or the polycondensation polymer while the centre of the moulded part has a temperature above the said softening temperature, and 1 to 60 wt. % of the mixture consists of reinforcing fibres with an average length of between 0.8 and 15 mm.
The moulded part surprisingly expands during the opening of the mould and a thermoplastic moulded part with a porous centre is obtained. The expanded moulded part obtained by the process according to the invention has a better specific modulus and strength than the known moulded part.
Partly opening the mould in this specification is understood to mean the opening of the mould over a certain path length until the distance between both mould halves is equal to the desired thickness of the moulded part.
In the case of moulded parts, the requirements with respect to stiffness and strength determine the moulded part""s thickness, and hence also its price. The so-called specific modulus and strength are used to enable comparison of the modulus and strength of materials for moulded parts. They are index numbers that are measures of the resistance offered by a moulded part to deformation and rupture, respectively, under the influence of a bending load per unit of density. The specific modulus and strength are used especially to compare the modulus and strength of materials having different densities when searching for the lightest material offering the greatest stiffness or strength for a particular shape. A detailed description of these index numbers is given in the xe2x80x9cMaterials Selector: guidelines for minimum weight designxe2x80x9d, Chapman and Hall, London.
Preferably, a converging nozzle is used in the process according to the invention, for this surprisingly causes better expansion of the moulded part during the opening of the mould. The use of a converging nozzle is known from WO-A-94/11177, but here a converging nozzle is used to obtain an orientation of the plastic and/or the fibre reinforcement in the moulded part, as a result of which this may possess a higher stiffness and strength in a particular direction. WO-A-94/11177 nowhere mentions the fact that a converging nozzle could lead to expansion of the moulded part, while the process according to the invention causes no or virtually no anisotropy in the moulded part.
A converging nozzle can be obtained with a conically ending nozzle, but also by for example placing a breaker plate (a plate with a number of openings) in front of the nozzle.
The merits of the invention are brought out very clearly if the process according to the invention is used for the production of dish-shaped moulded parts.
Such a moulded part possesses two dish surfaces, which are in this description understood to be the two, usually almost parallel, surfaces lying opposite one another, whose length and width are larger than the thickness of the moulded part lying between these surfaces. The dish surfaces need not only be flat, but may for instance also be curved or doubly curved.
Partly opening the mould only when at least one dish surface has cooled to below the softening temperature of the polyolefin or the polycondensation polymer results in a moulded part with at least one dish surface that is not porous, that is, with at least one surface free of pores. The presence of at least one dish surface that is not porous proves to result in an improvement of the specific modulus and strength with respect to a non-expanded moulded part. In addition, such a surface can suitably be painted.
Preferably, the mould is partly opened only when both dish surfaces have cooled down.
As a result, both dish surfaces are non-porous so that a moulded part with a sandwich structure is obtained. A sandwich structure gives a dish-shaped moulded part an extra high stiffness and strength. A weak point in sandwich structures is often the adhesion between the centre and the dish surface. In known sandwich structures this adhesion is often improved by using a so-called 3D fabric, which is understood to be a three-dimensional fibre structure in the form of a fabric. A characteristic of a 3D fabric is that some of the fibres that reinforce the centre continue into the dish surface, which promotes the adhesion between the centre and the dish surface. 3D fabrics and composites made therefrom are described by A. Schrauwers in xe2x80x9cKunststof Magazinexe2x80x9d, 1993, page 16. Drawbacks of using a 3D fabric are that it has to be made to measure beforehand and that it must separately be placed in the mould for each injection. A 3D fabric can moreover only be used in combination with thermosetting plastics with a very low viscosity. It has surprisingly been found that with the process according to the invention at least a portion of the reinforcing fibres is present partly in the centre and partly in the non-porous dish surface, as a result of which it is not necessary to use a 3D fabric. It has also been found that the fibres in the centre of the moulded part form a three-dimensional network and lie parallel to the surface at the moulded part""s dish surface. With this the process according to the invention for the first time offers the possibility of using the injection-moulding technique to produce (half) a fibre-reinforced sandwich from a thermoplastic fibre-reinforced plastic, in which the fibres of the centre continue into the surface. This structure contributes to the excellent specific modulus and strength, even if only one dish surface is non-porous.
The melt flow index (MFI) is in this description understood to be the melt flow index measured according to ISO 1133. For polypropylene the melt flow index is measured at 230xc2x0 C. under a weight of 2.16 kg.
The melt flow index of the polyolefin to be used in the process according to the invention is preferably higher than 30 g/10 min, even more preferably higher than 50 g/10 min. It has been found that at such a melt flow index better expansion of the moulded part takes place when the mould is opened. Preferably, the melt flow index is lower than 700 g/10 min.
The number average molecular mass (Mn) of the polycondensation polymers to be used in the process according to the invention is preferably higher than 5000 g/mol. The process proves to be effective for all the polycondensation polymers so far available. With the present polymerization technologies approximately 90,000 g/mol is the upper limit of the molecular mass of available polycondensation polymers. It may be expected that polycondensation polymers with higher molecular masses, if they become available, can be processed with the process according to the invention, up to a molecular mass of approximately 200,000 g/mol.
The xe2x80x98average fibre lengthxe2x80x99 is in this description understood to be the number average fibre length. This can be determined in the moulded part by measuring the length of the fibres with the aid of a light microscope after the polymer matrix has been removed, for example by burning out the polymer.
If the polymer composition contains glass fibres as reinforcing fibres, the polymer composition preferably contains 5-60 wt. %, more preferably 10-60 wt. %, of the glass fibres. If the polymer composition contains carbon fibres as reinforcing fibres, the polymer composition preferably contains 1-10 wt. %, preferably 2-7 wt. %, of the carbon fibres.
It has been found that such mixtures expand very well.
It has been found that the moulded part can easily expand to twenty times its original thickness. This results in a moulded part with a porosity of 95%. The porosity (P) of a moulded part is here and hereinafter understood to be:
xe2x80x83P=(d(0)xe2x88x92d(p))/d(0)*100(%),
where d(0) is the density before expansion and d(p) the density after expansion. An advantage of the process according to the invention is also that a porous moulded part can be obtained without having to use a chemical or physical foaming agent. A high porosity of the moulded part is advantageous with respect to achieving a high specific modulus and strength because these index numbers are inversely proportional to the density.
Preferably, the rate at which the mould is opened is chosen in dependence of the viscosity of the polyolefin or the Mn of the polycondensation polymers. At a high viscosity/Mn preferably a lower rate is chosen than at a low viscosity/Mn. The best results are obtained by adjusting the rate to the expansion rate of the moulded part so that, when the mould is opened, the moulded part continues to be pressed against the mould halves as a result of the expansion of the moulded part and the two dish surfaces accurately mirror the adjacent mould surfaces.
The mould is preferably opened at a rate that lies between 0.05 and 10 mm/sec, since this results in a moulded part having a high porosity and a surface without pores.
It has been found that these limits may vary depending on the amount of moisture in the dispersion. The greater the amount of moisture present in the dispersion, the faster or further the mould may be opened while still at least one smooth dish surface free of pores is obtained. A person skilled in the art will be able to easily determine the rate at which and the distance over which the mould is opened. Preferably, the amount of moisture in the dispersion is less than 5000 ppm. That way the occurrence of sink marks in the surface is avoided.
The invention also relates to a dish-shaped thermoplastic moulded part comprising at least a polyolefin or a polycondensation polymer and reinforcing fibres.
Such moulded parts are described in JP-A-5-17631. The moulded parts described in JP-A-5-17631 contain a polyolefin with a melt flow index of 30 g/10 min. or higher and more than 20 wt. % reinforcing fibres with a length of 5 mm or more.
A drawback of the moulded parts described in JP-A-5-17631 is that they have a specific modulus and specific strength that are too low for the fibre-reinforced polyolefins employed.
The aim of the invention is to provide a moulded part that does not possess the said drawback.
This aim is achieved in that the moulded part has a porous centre and at least one non-porous dish surface, 1 to 60 wt. % of the moulded part consists of reinforcing fibres with an average length of between 0.8 and 15 mm, at least a portion of which is present partly in the centre and partly in the non-porous dish surface, and the moulded part has a porosity of between 5 and 95 vol. %. Preferably between 10 and 90%, more preferably between 20 and 85%. The expanded moulded part according to the invention has a higher specific stiffness and strength than non-expanded moulded parts, which are for example described in JP-A-5-17631.
In the case of moulded parts with a porosity of more than 20 wt. % the anisotropy proves to be very low, while moulded parts with a porosity of less than 95 vol. % have a dish surface free of pores.
The polyolefin preferably has a melt flow index of at least 30 g/10 min.
The polyolefin can be chosen from the group comprising polyethylene and polypropylene or copolymers of ethylene and propylene. Preferably, the polyolefin contains polypropylene. The advantage of polypropylene is the high melting point, as a result of which the moulded part has a higher heat deflection temperature and a relatively low cost price.
Suitable polycondensation polymers are polycarbonate, polyester, polyamide, polyarylate, polyketone, polyimide, polyaramide, liquid crystal polymer (LCP), polyurethane and copolymers of such polycondensates. Preferably, polycondensation polymers are chosen from the group comprising polyamide 6, polyamide 6.6, polyamide 11, polyamide 12, polyethylene terephthalate, polybutylene terephthalate or copolymers thereof. The advantage of the latter group is the relatively low cost price.
Reinforcing fibres can be chosen from the group comprising carbon fibres, aramide fibres, metal fibres, glass fibres, ceramic fibres or mixtures hereof. Preferably, the moulded part according to the invention contains glass fibres or carbon fibres as the reinforcing fibres. Glass fibres have the advantage that they are cheap. Carbon fibres have the advantage that they have a high tensile strength.
Although the advantages of the invention are already achieved when at least one dish surface is not porous, preferably both dish surfaces are not porous. The moulded part then has a sandwich structure. The advantage of a sandwich structure is that the specific material properties are better than those of a moulded part that does not have this sandwich structure. As a result of this sandwich structure and the fact that a portion of the reinforcing fibres is partly in the centre and partly in the non-porous dish surface, the moulded part according to the invention preferably has a specific flexural modulus of at least 10.
Moulded parts according to the invention are preferably used in construction panels, elements/body panels for the automotive industry, the white goods and building industrie; splash shields, noise shields, fire walls, parcel shelves, mudguards, bonnets, dashboards, panels for washing machines, tumble dryers, caravans and planes, bumper beams, car doors, loading platforms, helmets, armour plates, insulation walls, both for heat and noise, containers, pallets, acoustic baffles, roof liners, transport containers and dashboards. The moulded parts according to the invention can also be used in bicycle parts, scooter parts and motorcycle parts. Especially in applications in which sound insulation is important the moulded parts according to the invention present a major advantage. In particular the use of the moulded part according to the invention presents advantages in products that have to meet sound-proofing requirements. It has been found that differences in density between the dish surface free of pores and the interior of the moulded parts according to the invention have a favourable effect on their sound-proofing properties.
The invention will be further elucidated with reference to the following examples.
Reinforcing fibres can be dispersed in a melt of a polyolefin or a condensation polymer by using as a starting material a chopped strand of continuous fibres that have been pultruded or sheathed with the thermoplastic in question.
In pultrusion a bundle of continuous fibres is spread out into individual fibres and drawn through an impregnation die, into which the melted thermoplastic is injected. As the fibres have been spread out, each filament is entirely wetted and impregnated by the melted thermoplastic. A smooth strand with a diameter of about 3 mm is drawn from the die and then cooled. Finally the strand is chopped into a granulate of the desired length (for instance 10-12 mm). The fibres are generally parallel to one another in the granulate, with each fibre being separately surrounded by thermoplastic. Pultruded fibres are marketed for example by Hoechst/PCI (Compec(copyright), Celstran(copyright)), Borealis (Nepol(copyright)) LNP/Kawasaki Steel (Verton(copyright)).
Sheathing continuous reinforcing fibres with thermoplastic without the individual fibres being wetted is in this description called Continuous Glass Sheathing (CGS). The advantage over pultrusion is the higher production rate (and hence lower costs). In the case of CGS granulate, too, the length of the glass is the same as the length of the granulate and the fibres are parallel to one another. Pultruded and sheathed fibres are easily dispersed in the melt of a thermoplastic in the melt zone of an extruder.
The melt flow index was measured according to ISO 1133, for polypropylene at 230xc2x0 C. and a weight of 2.16 kg.
The flexural modulus and flexural strength were determined according to ISO 178, with an 1/d ratio of 16.
The moulded part""s resistance to bending under the influence of a force is expressed in the specific modulus and the resistance to rupture in the specific strength. These specific quantities depend on the shape of the object in question. The specific modulus and strength of a dish-shaped moulded part are best approximated by those of a flat plate. The specific modulus of a flat plate is the quotient of the cube root of the flexural modulus and the density, the index number being obtained when the flexural modulus is expressed in kg/mm2 and the density in g/cm3. The specific strength of a plate is given by the quotient of the square root of the flexural stress and the density, the index number being obtained when the flexural stress is expressed in kg/mm2 and the density in g/cm3. A description of these index numbers is given in xe2x80x9cMaterials Selector: guidelines for minimum weight designxe2x80x9d, Chapman and Hall, London.
A Stork injection-moulding machine of type SX-3000-2100 was used in the tests. The employed screw was a general-purpose screw with a diameter of 72 mm, a length of 22D (feed/compression/pump: 12D/6D/4D). The depth of thread in the feed zone is 9.75 mm and that in the pump zone 5 mm. The compression ratio is 1.95.
The screw tip is a standard screw tip for the processing of PVC combined with a streamlined annular valve. The nose tip is a standard nose tip with a length of 117 mm and an internal diameter of 19 mm, which converges over a length of 10 mm to an ultimate diameter of 4 mm.
The material was injected into a flat plate mould (510xc3x97310 mm) via a central sprue.